Method for manufacturing semiconductor device

ABSTRACT

In a transistor including an oxide semiconductor film, a metal oxide film which has a function of preventing 1 electrification and covers a source electrode and a drain electrode is formed in contact with the oxide semiconductor film, and then, heat treatment is performed. Through the heat treatment, impurities such as hydrogen, moisture, a hydroxyl group, or hydride are intentionally removed from the oxide semiconductor film, whereby the oxide semiconductor film is highly purified. By providing the metal oxide film, generation of a parasitic channel on the back channel side of the oxide semiconductor film in the transistor is prevented.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/071,802, filed Mar. 25, 2011, now allowed, which claims the benefitof a foreign priority application filed in Japan as Serial No.2010-072256 on Mar. 26, 2010, both of which are incorporated byreference.

TECHNICAL FIELD

An embodiment of the present invention relates to a semiconductor deviceand a method for manufacturing the semiconductor device.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, and anelectro-optic device, a semiconductor circuit, and an electronic deviceare all semiconductor devices.

BACKGROUND ART

Attention has been focused on a technique for forming a transistor (alsoreferred to as a thin film transistor (TFT)) using a semiconductor thinfilm formed over a substrate having an insulating surface. Such atransistor is applied to a wide range of electronic devices such as anintegrated circuit (IC) or an image display device (display device). Asa semiconductor thin film applicable to the transistor, a silicon basedsemiconductor material is widely known. Moreover, an oxide semiconductorhas been attracting attention as another material.

For example, a transistor whose active layer includes an amorphous oxidecontaining indium (In), gallium (Ga), and zinc (Zn) and having anelectron carrier concentration of less than 10¹⁸/cm³ is disclosed (seePatent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528

DISCLOSURE OF INVENTION

However, the electric conductivity of an oxide semiconductor changeswhen deviation from the stoichiometric composition due to excess ordeficiency of oxygen or the like occurs, or hydrogen or moisture formingan electron donor enters the oxide semiconductor during a thin filmformation process. Such a phenomenon becomes a factor of variation inthe electric characteristics of a transistor including the oxidesemiconductor.

In view of the above problems, it is an object to provide asemiconductor device including an oxide semiconductor, which has stableelectric characteristics and high reliability.

In addition, it is an object to prevent generation of a parasiticchannel on the back channel side of an oxide semiconductor film.

In order to suppress variation in the electric characteristics of atransistor including an oxide semiconductor film, impurities such ashydrogen, moisture, a hydroxyl group, or hydride (also referred to as ahydrogen compound) which cause the variation are intentionally removedfrom the oxide semiconductor film. In addition, oxygen which is a maincomponent of an oxide semiconductor and is reduced in the step ofremoving the impurities is supplied. The oxide semiconductor film isthus highly purified and becomes electrically i-type (intrinsic).

An i-type (intrinsic) oxide semiconductor is an oxide semiconductorwhich is made to be i-type (intrinsic) or substantially i-type(intrinsic) by being highly purified by removing hydrogen, which is ann-type impurity, from the oxide semiconductor so that impurities thatare not a main component of the oxide semiconductor are contained aslittle as possible. In other words, a feature is that a highly purifiedi-type (intrinsic) oxide semiconductor or an oxide semiconductor closethereto is obtained not by adding impurities but by removing impuritiessuch as hydrogen or water as much as possible. This enables the Fermilevel (Ef) to be at the same level as the intrinsic Fermi level (Ei).

In a transistor including an oxide semiconductor film, an oxide layerhaving a function of preventing electrification is formed over and incontact with the oxide semiconductor film, and then, heat treatment isperformed.

The oxide layer having a function of preventing electrification ispreferably formed on the back channel side (the side opposite to thegate insulating film side) of the oxide semiconductor film, preferably,the highly purified oxide semiconductor film. In addition, the oxidelayer having a function of preventing electrification preferably has adielectric constant lower than that of the oxide semiconductor. Forexample, an oxide layer having a dielectric constant of 8 to 20inclusive may be used.

The oxide layer is thicker than the oxide semiconductor film. Forexample, provided that the thickness of the oxide semiconductor film is3 nm to 30 nm inclusive, the thickness of the oxide layer is preferablymore than 10 nm and more than or equal to the thickness of the oxidesemiconductor film.

A metal oxide can be used for the oxide layer. As the metal oxide, forexample, gallium oxide or gallium oxide to which indium or zinc is addedat 0.01 at. % to 5 at. % can be used.

The oxide semiconductor film is preferably highly purified byintentionally removing impurities such as hydrogen, moisture, a hydroxylgroup, or hydride (also referred to as a hydrogen compound) through theheat treatment. Hydrogen or a hydroxyl group which is an impurity can beeasily eliminated as water in the heat treatment.

The oxide semiconductor film and the metal oxide film containing oxygenare in contact with each other when being subjected to the heattreatment; thus, oxygen which is one of the main components of the oxidesemiconductor and is reduced in the step of removing impurities, can besupplied from the metal oxide film containing oxygen to the oxidesemiconductor film. Thus, the oxide semiconductor film is more highlypurified to become electrically i-type (intrinsic).

Further, in order to prevent entry of impurities such as moisture orhydrogen into the oxide semiconductor film after the heat treatment, aprotective insulating layer which prevents entry thereof from theoutside may be further formed over the metal oxide film.

The electric characteristics of the transistor including the highlypurified oxide semiconductor film, such as the threshold voltage and theoff-state current, have almost no temperature dependence. Further, thetransistor characteristics hardly change owing to light deterioration.

As described above, variation in the electric characteristics of thetransistor including the highly purified and electrically i-type(intrinsic) oxide semiconductor film is suppressed and the transistor iselectrically stable. Consequently, a highly reliable semiconductordevice including an oxide semiconductor, which has stable electriccharacteristics, can be provided.

The heat treatment is performed at a temperature of 250° C. to 650° C.inclusive, 450° C. to 600° C. inclusive, or less than the strain pointof a substrate. The heat treatment may be performed in an atmosphere ofnitrogen, oxygen, ultra-dry air (air in which the water content is 20ppm or less, preferably 1 ppm or less, more preferably 10 ppb or less),or a rare gas (argon, helium, or the like).

According to an embodiment of the structure of the invention disclosedin this specification, a semiconductor device includes: a gateelectrode, a gate insulating film which covers the gate electrode, anoxide semiconductor film in a region overlapping with the gate electrodeover the gate insulating film, a source electrode and a drain electrodewhich are in contact with the oxide semiconductor film, and a metaloxide film which is contact with the oxide semiconductor film and coversthe source electrode and the drain electrode.

In the above device, a gallium oxide film is preferably used as themetal oxide film. The gallium oxide film can be formed by a sputteringmethod, a CVD method, an evaporation method, or the like. The galliumoxide film has a band gap of about 4.9 eV and a light-transmittingproperty in the visible-light wavelength range, although depending onthe composition ratio of oxygen and gallium.

In this specification, gallium oxide is represented by GaOx (x>0) insome cases. For example, when GaOx has a crystal structure, Ga₂O₃ inwhich x is 1.5 is known.

In the above device, the metal oxide film is preferably a gallium oxidefilm containing indium or zinc at 0.01 at. % to 5 at. %. In addition,the difference between the band gap of the metal oxide film and the bandgap of the oxide semiconductor film is preferably less than 3.0 eV.

In addition, the oxide semiconductor film preferably contains indium andgallium.

In addition, in the above device, the source electrode and the drainelectrode preferably contain a conductive material whose work functionis 3.9 eV or more. The conductive material whose work function is 3.9 eVor more is preferably tungsten nitride or titanium nitride.

In addition, according to another embodiment of the invention disclosedin this specification, a method for manufacturing a semiconductor deviceincludes the steps of: forming a gate electrode over a substrate,forming a gate insulating film which covers the gate electrode, formingan oxide semiconductor film in a region overlapping with the gateelectrode with the gate insulating film interposed therebetween, forminga source electrode and a drain electrode over the oxide semiconductorfilm, forming a metal oxide film which covers the oxide semiconductorfilm, the source electrode, and the drain electrode, and performing heattreatment.

In the above method, a film containing gallium oxide is preferablyformed as the metal oxide film. In addition, a gallium oxide filmcontaining indium or zinc at 0.01 at. % to 5 at. % is preferably formedas the metal oxide film. In addition, the heat treatment is preferablyperformed at a temperature of 450° C. to 600° C.

According to an embodiment of the present invention, a metal oxide filmis formed over and in contact with an oxide semiconductor film, andthen, heat treatment is performed. Through the heat treatment,impurities such as hydrogen, moisture, a hydroxyl group, or hydride canbe intentionally removed from the oxide semiconductor film, whereby theoxide semiconductor film can be highly purified. Variation in theelectric characteristics of a transistor including the highly purifiedand electrically i-type (intrinsic) oxide semiconductor film issuppressed and the transistor is electrically stable.

Therefore, according to an embodiment of the present invention, atransistor having stable electric characteristics can be manufactured.

In addition, according to an embodiment of the present invention, asemiconductor device including a transistor, which has favorableelectric characteristics and high reliability, can be manufactured.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1D are diagrams illustrating an embodiment of asemiconductor device and a method for manufacturing the semiconductordevice.

FIGS. 2A to 2C are diagrams each illustrating an embodiment of asemiconductor device.

FIG. 3 is a diagram illustrating an embodiment of a semiconductordevice.

FIG. 4 is a diagram illustrating an embodiment of a semiconductordevice.

FIG. 5 is a diagram illustrating an embodiment of a semiconductordevice.

FIGS. 6A and 6B are diagrams illustrating an embodiment of asemiconductor device.

FIGS. 7A and 7B are diagrams illustrating an electronic device.

FIGS. 8A to 8F are diagrams each illustrating an electronic device.

FIG. 9A is a model diagram illustrating a stacked-layer structure ofdielectrics and FIG. 9B is an equivalent circuit diagram.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. Therefore, the present inventionis not construed as being limited to the description of the embodimentsbelow.

Note that the ordinal numbers such as “first” and “second” in thisspecification are used for convenience and do not denote the order ofsteps and the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify theinvention.

Embodiment 1

In this embodiment, an embodiment of a semiconductor device and a methodfor manufacturing the semiconductor device will be described withreference to FIGS. 1A to 1D. In this embodiment, a transistor includingan oxide semiconductor film will be described as an example of thesemiconductor device.

As illustrated in FIG. 1D, a transistor 410 includes, over a substrate400 having an insulating surface, a gate electrode 401, a gateinsulating film 402, an oxide semiconductor film 403, a source electrode405 a, and a drain electrode 405 b. A metal oxide film 407 having afunction of preventing electrification on the back channel side of theoxide semiconductor film 403 is provided over the oxide semiconductorfilm 403.

FIGS. 1A to 1D illustrate an example of a method for manufacturing thetransistor 410.

First, a conductive film is formed over the substrate 400 having aninsulating surface, and then, the gate electrode 401 is formed by afirst photolithography step. Note that a resist mask may be formed by aninkjet method. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced.

Although there is no particular limitation on a substrate which can beused as the substrate 400 having an insulating surface, it is necessarythat the substrate have at least enough heat resistance to a heattreatment to be performed later. For example, a substrate such as aglass substrate, a ceramic substrate, a quartz substrate, or a sapphiresubstrate can be used. In addition, a single crystal semiconductorsubstrate or a polycrystalline semiconductor substrate of silicon,silicon carbide, or the like, a compound semiconductor substrate ofsilicon germanium or the like, an SOI substrate, or the like can be usedas long as the substrate has an insulating surface. The transistor 410may be provided over such a substrate.

Further, a flexible substrate may be used as the substrate 400. In thecase where a flexible substrate is used, the transistor 410 includingthe oxide semiconductor film 403 may be directly formed over a flexiblesubstrate. Alternatively, the transistor 410 including the oxidesemiconductor film 403 may be formed over a manufacturing substrate, andthen, the transistor 410 may be separated from the manufacturingsubstrate and transferred to a flexible substrate. Note that in order toseparate the transistor from the manufacturing substrate and transfer itto the flexible substrate, a separation layer may be provided betweenthe manufacturing substrate and the transistor including the oxidesemiconductor film.

Note that an insulating film serving as a base film may be providedbetween the substrate 400 and the gate electrode 401. The base film hasa function of preventing diffusion of impurity elements from thesubstrate 400, and can be formed using one or more of a silicon nitridefilm, a silicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

In addition, the gate electrode 401 can be formed to have a single-layerstructure or a stack-layer structure using a metal material such asmolybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium,or scandium, or an alloy material which contains any of these materialsas a main component.

Next, the gate insulating film 402 is formed over the gate electrode401. The gate insulating film 402 can be formed using one or more of asilicon oxide film, a silicon nitride film, a silicon oxynitride film, asilicon nitride oxide film, an aluminum oxide film, an aluminum nitridefilm, an aluminum oxynitride film, an aluminum nitride oxide film, and ahafnium oxide film by a plasma CVD method, a sputtering method, or thelike.

The oxide semiconductor film 403 in this embodiment is formed using anintrinsic (i-type) or substantially intrinsic (i-type) oxidesemiconductor from which impurities are removed and which is highlypurified so as to contain an impurity that serves as a carrier donor andis a substance other than the main component of the oxide semiconductoras little as possible. Specifically, the hydrogen concentration in theoxide semiconductor film 403 is 5×10¹⁹ atoms/cm³ or less, preferably5×10¹⁸ atoms/cm³ or less, more preferably 5×10¹⁷ atoms/cm³ or less. Notethat the hydrogen concentration in the oxide semiconductor film 403 ismeasured by secondary ion mass spectroscopy (SIMS). In the oxidesemiconductor film 403 which is highly purified by sufficiently reducingthe hydrogen concentration and in which defect levels in an energy gapdue to oxygen deficiency are reduced by supplying a sufficient amount ofoxygen, the carrier concentration is less than 1×10¹²/cm³, preferablyless than 1×10¹¹/cm³, more preferably less than 1.45×10¹⁰/cm³. Forexample, the off-state current (here, current per micrometer (μm) ofchannel width) at room temperature (25° C.) is 100 zA (1 zA(zeptoampere) is 1×10⁻²¹ A) or less, preferably 10 zA or less. Thetransistor 410 with excellent off-state current characteristics can beobtained with the use of an i-type (intrinsic) or substantially i-typeoxide semiconductor.

Such a highly purified oxide semiconductor is highly sensitive to aninterface state or interface charge; thus, an interface between theoxide semiconductor film and the gate insulating film is important.Thus, the gate insulating film that is to be in contact with the highlypurified oxide semiconductor needs to have high quality.

For the method for manufacturing the gate insulating film, ahigh-density plasma CVD method using microwaves (e.g., with a frequencyof 2.45 GHz) is preferably employed because an insulating layer which isformed can be dense and can have high breakdown voltage and highquality. When the highly purified oxide semiconductor and thehigh-quality gate insulating film are in close contact with each other,the interface state density can be reduced and favorable interfacecharacteristics can be obtained.

Needless to say, a different film formation method such as a sputteringmethod or a plasma CVD method can be used as long as a high-qualityinsulating film can be formed as the gate insulating film. In addition,an insulating film can be used, whose film quality as the gateinsulating film and characteristics of an interface with the oxidesemiconductor are modified by heat treatment performed after formationof the oxide semiconductor film. In any case, any insulating film can beused as long as film quality as the gate insulating film is favorable,interface state density with the oxide semiconductor is decreased, and afavorable interface can be formed.

In order that hydrogen, a hydroxyl group, and moisture are contained aslittle as possible in the gate insulating film 402 and the oxidesemiconductor film, it is preferable that the substrate 400 over whichthe gate electrode 401 is formed or the substrate 400 over which filmsup to the gate insulating film 402 are formed be preheated in apreheating chamber of a sputtering apparatus as pretreatment for theformation of the oxide semiconductor film, so that impurities such ashydrogen or moisture absorbed onto the substrate 400 are eliminated andremoved. As an evacuation unit provided in the preheating chamber, acryopump is preferable. Note that this preheating treatment can beomitted. In addition, this preheating treatment may be performed on thesubstrate 400 over which films up to the source electrode 405 a and thedrain electrode 405 b are formed (before forming the metal oxide film407) in a later step in a similar manner.

Next, an oxide semiconductor film having a thickness of 3 nm to 30 nminclusive is formed over the gate insulating film 402 by a sputteringmethod. The thickness in the above range is preferable because when thethickness of the oxide semiconductor film is too large (for example,when the thickness is 50 nm or more), the transistor might be normallyon.

Note that before the oxide semiconductor film is formed by a sputteringmethod, powdery substances (also referred to as particles or dusts)which are attached on a surface of the gate insulating film 402 arepreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which, without application of a voltage to a target side, anRF power source is used for application of a voltage to a substrate sidein an argon atmosphere to generate plasma in the vicinity of thesubstrate and modify a surface. Note that instead of an argonatmosphere, a nitrogen atmosphere, a helium atmosphere, an oxygenatmosphere, or the like may be used.

As an oxide semiconductor used for the oxide semiconductor film, thefollowing oxide semiconductors can be used: a four-component metal oxidesuch as an In—Sn—Ga—Zn—O based oxide semiconductor; a three-componentmetal oxide such as an In—Ga—Zn—O based oxide semiconductor, anIn—Sn—Zn—O based oxide semiconductor, an In—Al—Zn—O based oxidesemiconductor, a Sn—Ga—Zn—O based oxide semiconductor, an Al—Ga—Zn—Obased oxide semiconductor, or a Sn—Al—Zn—O based oxide semiconductor, atwo-component metal oxide such as an In—Zn—O based oxide semiconductor,a Sn—Zn—O based oxide semiconductor, an Al—Zn—O based oxidesemiconductor, a Zn—Mg—O based oxide semiconductor, a Sn—Mg—O basedoxide semiconductor, an In—Mg—O based oxide semiconductor, or an In—Ga—Obased oxide semiconductor, an In—O based oxide semiconductor, a Sn—Obased oxide semiconductor, or a Zn—O based oxide semiconductor, and thelike. Further, SiO₂ may be contained in the above oxide semiconductor.Note that here, for example, an In—Ga—Zn—O based oxide semiconductormeans an oxide semiconductor containing indium (In), gallium (Ga), andzinc (Zn) and there is no particular limitation on the compositionratio. The In—Ga—Zn—O based oxide semiconductor may contain an elementother than In, Ga, and Zn.

In addition, for the oxide semiconductor film, a thin film of a materialrepresented by the chemical formula, InMO₃(ZnO)_(m) (m>0), can be used.Here, M represents one or more metal elements selected from Ga, Al, Mn,and Co. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, orthe like.

When an In—Ga—Zn—O based material is used as an oxide semiconductor, forexample, an oxide target having a composition ratio of In₂O₃: Ga₂O₃:ZnO=1:1:1 [molar ratio] can be used. In addition, without limitation tothe material and the component of this target, for example, an oxidetarget having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio]may be used.

When an In—Zn—O based material is used as an oxide semiconductor, atarget to be used has a composition ratio of In:Zn=50:1 to 1:2 in anatomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferablyIn:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molarratio), more preferably In:Zn=15:1 to 1.5:1 in an atomic ratio(In₂O₃:ZnO=15:2 to 3:4 to in a molar ratio). For example, in a targetused for formation of an In—Zn—O based oxide semiconductor which has anatomic ratio of In:Zn:O=X:Y:Z, an inequality of Z>1.5X+Y is satisfied.

Furthermore, the filling rate of the target is 90% to 100% inclusive,preferably 95% to 99.9% inclusive. With the use of the target with ahigh filling rate, a dense oxide semiconductor film can be formed.

In this embodiment, the oxide semiconductor film is formed by asputtering method with the use of an In—Ga—Zn—O based oxide target.Alternatively, the oxide semiconductor film can be formed by asputtering method in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere containing a rare gas and oxygen.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, a hydroxyl group, or hydride are removed, as asputtering gas used when the oxide semiconductor film is formed.

For the formation of the oxide semiconductor film, the substrate 400 isset in a film formation chamber at reduced pressure and the substratetemperature is set to 100° C. to 600° C. inclusive, preferably 200° C.to 400° C. inclusive. When film formation is performed while thesubstrate 400 is heated, the concentration of impurities contained inthe oxide semiconductor film can be reduced. In addition, damage bysputtering can be reduced. Then, residual moisture in the film formationchamber is removed, a sputtering gas from which hydrogen and moistureare removed is introduced, and the above-described target is used, sothat the oxide semiconductor film is formed over the substrate 400. Inorder to remove moisture remaining in the film formation chamber, anentrapment vacuum pump such as a cryopump, an ion pump, or a titaniumsublimation pump is preferably used. The evacuation unit may be a turbopump provided with a cold trap. In the film formation chamber which isevacuated with a cryopump, for example, a hydrogen atom, a compoundcontaining a hydrogen atom, such as water (H₂O), (preferably, also acompound containing a carbon atom), and the like are removed, wherebythe concentration of impurities in the oxide semiconductor film formedin the film formation chamber can be reduced.

As one example of the film formation condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, the electricpower of a direct-current (DC) power source is 0.5 kW, and theatmosphere is an oxygen atmosphere (the proportion of the oxygen flow is100%). Note that a pulse direct-current power source is preferably used,in which case powder substances (also referred to as particles or dusts)that are generated in film formation can be reduced and the filmthickness can be uniform.

Next, the oxide semiconductor film is processed into an island-shapedoxide semiconductor film 441 by a second photolithography step (see FIG.1A). A resist mask for forming the island-shaped oxide semiconductorfilm 441 may be formed by an inkjet method. Formation of the resist maskby an inkjet method needs no photomask; thus, manufacturing cost can bereduced. In the above manner, the oxide semiconductor film in a regionoverlapping with the gate electrode can be formed over the gateinsulating film.

Note that the etching of the oxide semiconductor film may be dryetching, wet etching, or both dry etching and wet etching. As an etchantused for wet etching of the oxide semiconductor film, a mixed solutionof phosphoric acid, acetic acid, and nitric acid, or the like can beused, for example. In addition, ITO07N (produced by KANTO CHEMICAL CO.,INC.) may be used.

Next, a conductive film for forming the source electrode and the drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed over the gate insulatingfilm 402 and the oxide semiconductor film 441. As the conductive filmfor forming the source electrode and the drain electrode, for example, ametal film containing an element selected from Al, Cr, Cu, Ta, Ti, Mo,and W, a metal nitride film containing any of the above elements as itscomponent (e.g., a titanium nitride film, a molybdenum nitride film, ora tungsten nitride film), or the like can be used. Alternatively, a filmof a high-melting-point metal such as Ti, Mo, or W or a metal nitridefilm thereof (e.g., a titanium nitride film, a molybdenum nitride film,or a tungsten nitride film) may be formed over or/and below a metal filmsuch as an Al film or a Cu film. Alternatively, the conductive film forforming the source electrode and the drain electrode may be formed usinga conductive metal oxide. As the conductive metal oxide, indium oxide(In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), an indium oxide-tin oxidealloy (In₂O₃—SnO₂; abbreviated to ITO), an indium oxide-zinc oxide alloy(In₂O₃—ZnO), or any of these metal oxide materials in which siliconoxide is contained can be used.

Note that it is preferable that a material of the source electrode andthe drain electrode be selected in consideration of the electronaffinity of the oxide semiconductor and the electron affinity of themetal oxide film. That is, when the work function of the material of thesource electrode and the drain electrode is W [eV], the electronaffinity of the oxide semiconductor is φ₁ [eV], and the electronaffinity of the metal oxide film is φ₂ [eV], it is preferable that thefollowing inequality be satisfied: φ₂+0.4<W<(φ₁+0.5), preferably(φ₂+0.9)<W<(φ₁+0.4). For example, when a material whose electronaffinity is 4.5 eV and a material whose electron affinity is 3.5 eV areused for the oxide semiconductor and the metal oxide film respectively,a metal or a metal compound whose work function is more than 3.9 eV andless than 5.0 eV, preferably more than 4.4 eV and less than 4.9 eV ispreferably used for the material of the source electrode and the drainelectrode. Thus, in the transistor 410, electrons can be prevented frombeing injected into the metal oxide film 407 from the source electrode405 a and the drain electrode 405 b, and generation of leakage currentcan be suppressed. In addition, favorable electric characteristics canbe obtained at a junction between the oxide semiconductor film and thesource and drain electrodes. For a material having such a work function,molybdenum nitride, tungsten nitride, or the like can be given, forexample. These materials are preferable because they are also excellentin heat resistance. Note that from the above inequality, an inequalityφ₂<(φ₁+0.1), preferably an inequality φ₂<(φ₁−0.5) is derived, but it ismore preferable that an inequality φ₂<(φ₁−0.9) be satisfied.

A resist mask is formed over the conductive film for forming the sourceelectrode and the drain electrode by a third photolithography step.Etching is selectively performed, so that the source electrode 405 a andthe drain electrode 405 b are formed. Then, the resist mask is removed(see FIG. 1B).

Light exposure at the time of the formation of the resist mask in thethird photolithography step may be performed using ultraviolet light,KrF laser light, or ArF laser light. The channel length L of thetransistor to be formed later is determined by a distance between alower end of the source electrode 405 a and a lower end of the drainelectrode 405 b that are adjacent to each other over the oxidesemiconductor film 441. In the case where light exposure is performedfor a channel length L of less than 25 nm, the light exposure at thetime of the formation of the resist mask in the third photolithographystep may be performed using extreme ultraviolet light having anextremely short wavelength of several nanometers to several tens ofnanometers. In the light exposure using extreme ultraviolet light, theresolution is high and the focus depth is large. For these reasons, thechannel length L of the transistor to be formed later can be 10 nm to1000 nm inclusive, and the circuit can operate at higher speed.

In order to reduce the number of photomasks and the number of steps inphotolithography, an etching step may be performed with the use of aresist mask formed using a multi-tone mask which is a light-exposuremask through which light is transmitted to have a plurality ofintensities. A resist mask formed with the use of a multi-tone mask hasa plurality of thicknesses and further can be changed in shape byetching; therefore, the resist mask can be used in a plurality ofetching steps for processing into different patterns. Therefore, aresist mask corresponding to at least two kinds of different patternscan be formed by using one multi-tone mask. Thus, the number oflight-exposure masks can be reduced and the number of correspondingphotolithography steps can also be reduced, whereby simplification of aprocess can be realized.

Note that it is preferable that etching conditions be optimized so asnot to etch and divide the oxide semiconductor film 441 when theconductive film for forming the source electrode and the drain electrodeis etched. However, it is difficult to obtain etching conditions inwhich only the conductive film is etched and the oxide semiconductorfilm 441 is not etched at all. In some cases, only part of the oxidesemiconductor film 441 is etched when the conductive film is etched, sothat an oxide semiconductor film having a groove portion (a recessedportion) is formed.

In this embodiment, since a Ti film is used as the conductive film forforming the source electrode and the drain electrode and an In—Ga—Zn—Obased oxide semiconductor is used as the oxide semiconductor film 441,ammonium hydrogen peroxide (31 wt % hydrogen peroxide: 28 wt % ammoniawater: water=5:2:2) is used as an etchant.

Next, by plasma treatment using a gas such as N₂O, N₂, or Ar, hydrogen,water, or the like adsorbed to a surface of an exposed portion of theoxide semiconductor film may be removed. In the case where plasmatreatment is performed, the metal oxide film 407 in contact with part ofthe oxide semiconductor film 441 is preferably formed following theplasma treatment without exposure to the air.

Then, the metal oxide film 407 is formed, which covers the sourceelectrode 405 a and the drain electrode 405 b and is in contact withpart of the oxide semiconductor film 441 (see FIG. 1C). Note that thethickness of the metal oxide film 407 is made larger than that of theoxide semiconductor film 441. The metal oxide film 407 is in contactwith the back channel side of the oxide semiconductor film 441, that is,part of the oxide semiconductor film 441 which is between the sourceelectrode 405 a and the drain electrode 405 b. The metal oxide film 407is a film removing charges accumulated at the interface with the oxidesemiconductor film 441.

A positive charge is moved from the source electrode 405 a or the drainelectrode 405 b to the oxide semiconductor film due to chargesaccumulated in the source electrode 405 a or the drain electrode 405 b,so that the interface at the back channel side of the oxidesemiconductor film might be electrified. In particular, when theelectric conductivity of an oxide semiconductor film and the electricconductivity of a material layer in contact with a back channel side ofthe oxide semiconductor film are different from each other, a chargeflows to the oxide semiconductor film, and the charge is trapped at theinterface and is bonded to hydrogen in the oxide semiconductor film tobe a donor center of the interface. Consequently, there is a problem inthat characteristics of a transistor vary. Therefore, both reduction ofhydrogen and prevention of electrification in the oxide semiconductorfilm are important.

The difference between the band gap of the oxide semiconductor film andthe band gap of the metal oxide film is preferably less than 3 eV. Forexample, in the case where an In—Ga—Zn—O based oxide semiconductor isused as the oxide semiconductor film and silicon oxide or aluminum oxideis used as the metal oxide film, since the band gap of the In—Ga—Zn—Obased oxide semiconductor is 3.15 eV, and the band gap of silicon oxideor aluminum oxide is 8 eV, the above-described problem might occur.Further, when a film containing nitride (such as a silicon nitride film)is used instead of the metal oxide film, the electric conductivity ofthe oxide semiconductor film might change owing to a contact between thefilm containing nitride and the oxide semiconductor film.

The metal oxide film 407 is preferably a film having a property ofremoving a positive charge immediately, when the back channel side ispositively charged. Note that as a material of the metal oxide film 407,a material whose hydrogen content is less than or equal to that of theoxide semiconductor film or is not larger than that of the oxidesemiconductor film by an order of magnitude or more and whose energy gapis more than or equal to that of a material of the oxide semiconductorfilm is preferably used.

In an embodiment of the present invention, the case where gallium oxide,for example, GaOx (x>0) is used as the metal oxide film is described.The physical property values of gallium oxide are as follows. Forexample, the band gap is 3.0 eV to 5.2 eV (e.g., 4.9 eV), the dielectricconstant is 8 to 20, and the electron affinity is 3.5 eV. The physicalproperty values of the In—Ga—Zn—O based oxide semiconductor are asfollows. For example, the band gap is 3.15 eV, the dielectric constantis 15, and the electron affinity is 3.5 eV. As described here, thedifference between the physical property values of gallium oxide and thephysical property values of the In—Ga—Zn—O based oxide semiconductor issmall, which is preferable. Since gallium oxide has a wide band gap ofabout 4.9 eV, it has light-transmitting properties in a visible-lightwavelength range. Further, it is preferable that gallium oxide be usedas the metal oxide film because contact resistance between an In—Ga—Zn—Obased oxide semiconductor film and a gallium oxide film can be reduced.In the case where gallium oxide is used as the metal oxide film, anIn—Ga—O based oxide semiconductor or a Ga—Zn—O based oxide semiconductormay be used as the oxide semiconductor material in addition to theIn—Ga—Zn—O based oxide semiconductor.

As described above, with the use of the metal oxide film having afunction of preventing electrification, accumulation of charges at theback channel side of the oxide semiconductor film can be suppressed.Further, even when the back channel side of the oxide semiconductor filmis positively charged, a positive charge can be removed immediately withthe metal oxide film provided over a top surface of the oxidesemiconductor film. Furthermore, with the use of the metal oxide film407, generation of a parasitic channel on the back channel side of theoxide semiconductor film 403 can be prevented. Consequently, variationin the electric characteristics of the oxide semiconductor film, such asthe electric conductivity, can be suppressed, whereby the reliability ofthe transistor can be improved.

A gallium oxide film is preferably used as the metal oxide film. Thegallium oxide film can be formed by a sputtering method, a CVD method,an evaporation method, or the like. The gallium oxide film has a bandgap of about 4.9 eV and has a light-transmitting property in thevisible-light wavelength range, although depending on the compositionratio of oxygen and gallium.

In this specification, gallium oxide is represented by GaOx (x>0) insome cases. For example, when GaOx has a crystal structure, Ga₂O₃ inwhich x is 1.5 is known.

In this embodiment, a gallium oxide film obtained by a sputtering methodusing a pulse direct current (DC) power source is used as the metaloxide film 407. Note that a gallium oxide target is preferably used as atarget used for a sputtering method. The electric conductivity of themetal oxide film 407 may be appropriately adjusted by adding indium orzinc to the metal oxide film 407, in accordance with the electricconductivity of the oxide semiconductor film which is used. For example,a film containing indium or zinc at 0.01 at. % to 5 at. % is formed by asputtering method using a target obtained by adding indium or zinc togallium oxide. When the electric conductivity of the metal oxide film407 is improved and is brought close to the electric conductivity of theoxide semiconductor film 403 by adding indium or zinc, the accumulatedcharges can be more reduced.

In particular, in the case where an In—Ga—Zn—O film is used as the oxidesemiconductor film, since the In—Ga—Zn—O film contains a gallium elementwhich is common with GaOx used as the metal oxide film 407, a materialof the oxide semiconductor film and a material of the metal oxide filmare compatible with each other.

The metal oxide film 407 is preferably formed by using a method withwhich impurities such as water or hydrogen do not enter the metal oxidefilm 407. When hydrogen is contained in the metal oxide film 407, entryof hydrogen into the oxide semiconductor film or extraction of oxygenfrom the oxide semiconductor film by hydrogen is caused; thus, a backchannel of the oxide semiconductor film might have low resistance(n-type conductivity) and a parasitic channel might be formed.Therefore, it is important that a formation method in which hydrogen isnot used is employed such that the metal oxide film 407 contains aslittle hydrogen as possible.

In this embodiment, as the metal oxide film 407, a gallium oxide filmhaving a thickness of more than 10 nm and more than or equal to that ofthe oxide semiconductor film 441 is formed by a sputtering method. Thisis because the metal oxide film 407 can remove a charge efficiently byincreasing the thickness of the metal oxide film 407 to be more than orequal to that of the oxide semiconductor film 441 in such a manner. Thesubstrate temperature at the time of film formation is room temperatureto 300° C. inclusive. The gallium oxide film can be formed by asputtering method in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere containing a rare gas and oxygen.

In order to remove residual moisture from the film formation chamber ofthe metal oxide film 407 in a manner similar to that of the formation ofthe oxide semiconductor film, an entrapment vacuum pump (such as acryopump) is preferably used. When the metal oxide film 407 is formed inthe film formation chamber evacuated using a cryopump, the concentrationof impurities contained in the metal oxide film 407 can be reduced. Inaddition, as an evacuation unit for removing the residual moisture fromthe film formation chamber of the metal oxide film 407, a turbo pumpprovided with a cold trap may be used.

It is preferable to use a high-purity gas from which impurities such ashydrogen, water, a hydroxyl group, or hydride are removed as asputtering gas when the metal oxide film 407 is formed.

The metal oxide film 407 may cover at least the channel formation regionof the oxide semiconductor film, the source electrode 405 a, and thedrain electrode 405 b. If needed, the metal oxide film 407 may beselectively removed. Note that known wet etching or known dry etchingcan be used for etching of the gallium oxide film used in thisembodiment. For example, wet etching is performed using a hydrofluoricacid solution or a nitric acid solution.

Next, the oxide semiconductor film 441 part of which (the channelformation region) is in contact with the metal oxide film 407 issubjected to heat treatment (see FIG. 1C).

The heat treatment is performed at a temperature of 250° C. to 650° C.inclusive, preferably 450° C. to 600° C. inclusive, or less than thestrain point of the substrate. For example, the substrate is introducedinto an electric furnace which is one of heat treatment apparatuses, andheat treatment is performed on the oxide semiconductor film at 450° C.for 1 hour in a nitrogen atmosphere.

Further, the heat treatment apparatus is not limited to an electricfurnace, and a device for heating an object to be processed by heatconduction or heat radiation from a heating element such as a resistanceheating element may be used. For example, a rapid thermal annealing(RTA) apparatus such as a gas rapid thermal annealing (GRTA) apparatusor a lamp rapid thermal annealing (LRTA) apparatus can be used. An LRTAapparatus is an apparatus for heating an object to be processed byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high pressure sodium lamp, or a high pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, an inert gas whichdoes not react with an object to be processed by heat treatment, such asnitrogen or a rare gas like argon, is used. Note that in the case wherea GRTA apparatus is used as the heat treatment apparatus, the substratemay be heated in an inert gas heated to a high temperature of 650° C. to700° C. because the heat treatment time is short.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is 20 ppm or less,preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas(argon, helium, or the like). Note that it is preferable that water,hydrogen, or the like be not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. Alternatively, nitrogen, oxygen,or a rare gas which is introduced into the heat treatment apparatus hasa purity of 6N (99.9999%) or more, preferably 7N (99.99999%) or more(that is, the impurity concentration is 1 ppm or less, preferably 0.1ppm or less).

In addition, the oxide semiconductor film and the metal oxide film 407containing oxygen are in contact with each other when being subjected tothe heat treatment; thus, oxygen which is one of the main components ofthe oxide semiconductor and is reduced in the step of removingimpurities, can be supplied from the metal oxide film 407 containingoxygen to the oxide semiconductor film. Consequently, a charge trappingcenter in the oxide semiconductor film can be reduced. Through the abovesteps, the oxide semiconductor film 403 which is highly purified and ismade electrically i-type (intrinsic) can be obtained. In addition,impurities are removed from the metal oxide film 407 at the same time bythis heat treatment, and the metal oxide film 407 can be highlypurified.

The highly purified oxide semiconductor film 403 includes extremely fewcarriers derived from a donor. The carrier concentration of the oxidesemiconductor film 403 is less than 1×10¹⁴/cm³, preferably less than1×10¹²/cm³, more preferably less than 1×10¹¹/cm³.

Through the above-described steps, the transistor 410 is formed (seeFIG. 1D). The transistor 410 is a transistor including the oxidesemiconductor film 403 which is highly purified and from whichimpurities such as hydrogen, moisture, a hydroxyl group, or hydride(also referred to as a hydrogen compound) are intentionally removed.Therefore, variation in the electric characteristics of the transistor410 is suppressed and the transistor 410 is electrically stable.

The other heat treatment may be performed in addition to the above heattreatment. For example, heat treatment (first heat treatment) may beperformed after the oxide semiconductor film 441 is formed, and heattreatment (second heat treatment) may be further performed after themetal oxide film 407 is formed. In this case, the first heat treatmentcan be treatment in which heating is performed in an inert gasatmosphere and cooling is performed in an oxygen atmosphere (in anatmosphere at least containing oxygen), for example. When such firstheat treatment is used, dehydration and supply of oxygen can befavorably performed on the oxide semiconductor film.

After the second heat treatment, heat treatment may be furtherperformed. For example, heat treatment may be performed at 100° C. to200° C. inclusive in the air for 1 hour to 30 hours inclusive. This heattreatment may be performed at a fixed heating temperature.Alternatively, the following change in the heating temperature may beconducted plural times repeatedly: the heating temperature is increasedfrom a room temperature to a temperature of 100° C. to 200° C. inclusiveand then decreased to a room temperature.

In addition, in the transistor 410 including the oxide semiconductorfilm 403, relatively high field-effect mobility can be obtained, wherebyhigh-speed operation is possible. Consequently, when the abovetransistor is used in a pixel portion, high-quality images can beobtained. In addition, since a driver circuit portion and a pixelportion each including the transistor including the highly purifiedoxide semiconductor film can be formed over one substrate, the number ofcomponents of the semiconductor device can be reduced.

In the transistor 410 including the metal oxide film 407, generation ofa parasitic channel on the back channel side of the oxide semiconductorfilm 403 can be prevented. By preventing the generation of a parasiticchannel on the back channel side of the oxide semiconductor film 403 inthe transistor 410, variation in the threshold voltage can besuppressed, whereby the reliability of the transistor can be improved.

In the transistor 410 illustrated in FIG. 1D, two dielectric layers, theoxide semiconductor film 403 and the metal oxide film 407, are providedin contact with each other. In the case where two different dielectriclayers are stacked, the stacked two layers can be expressed as in amodel diagram in FIG. 9A when the dielectric constant, the electricconductivity, and the thickness of a first layer (the oxidesemiconductor film 403 in the transistor 410) are set to ∈₁, σ₁, and d₁respectively, and the dielectric constant, the electric conductivity,and the thickness of a second layer (the metal oxide film 407 in thetransistor 410) are set to ∈₂, σ₂, and d₂ respectively. Note that inFIG. 9A, S represents an area. The model diagram in FIG. 9A can bereplaced with an equivalent circuit in FIG. 9B. C₁, G₁, C₂, and G₂ inthe drawing represent the capacitance value of the first layer, theresistance value of the first layer, the capacitance value of the secondlayer, and the resistance value of the second layer, respectively. Here,it is considered that in the case where a voltage V is applied to thetwo layers, a charge Q expressed by the following equation (1) isaccumulated at the interface between the two layers after t seconds.

$\begin{matrix}{Q = {\frac{{C_{2}G_{1}} - {C_{1}G_{2}}}{G_{1} + G_{2}}V \times \left\{ {1 - {\exp \left( {{- \frac{G_{1} + G_{2}}{C_{1} + C_{2}}}t} \right)}} \right\}}} & (1)\end{matrix}$

In the transistor 410 illustrated in FIG. 1D, the interface at which thecharge Q is accumulated corresponds to the back channel side of theoxide semiconductor film 403. The charge Q accumulated at the interfaceon the back channel side can be decreased by appropriately setting thedielectric constant, the electric conductivity, or the thickness of themetal oxide film 407.

Here, the equation (1) is modified into equations (2) and (3).

$\begin{matrix}{\mspace{79mu} {Q = {\left( {1 - \frac{\tau_{1}}{\tau_{2}}} \right)C_{2}V_{2} \times \left\{ {1 - {\exp \left( {- \frac{t}{\tau_{i}}} \right)}} \right\}}}} & (2) \\{\mspace{79mu} {V_{2} = {\frac{G_{1}}{G_{1} + G_{2}}{V\left( {{{{Not}\mspace{14mu} {that}\mspace{14mu} C_{1}} = {\frac{ɛ_{1}}{d_{1}}S}},{C_{2} = {\frac{ɛ_{2}}{d_{2}}S}},{G_{1} = {\frac{\sigma_{1}}{d_{1}}S}},{G_{2} = {\frac{\sigma_{2}}{d_{2}}S}},{\tau_{1} = \frac{ɛ_{1}}{\sigma_{1}}},{\tau_{2} = \frac{ɛ_{2}}{\sigma_{2}}},{\tau_{i} = \frac{C_{1} + C_{2}}{G_{1} + G_{2}}}} \right)}}}} & (3)\end{matrix}$

From the equations (2) and (3), four conditions (A) to (D) can beassumed in order to decrease the charge Q.

Condition (A): τ_(i) is extremely large.Condition (B): V₂ is close to zero, that is, G₂ is much larger than G₁.Condition (C): C₂ is close to zero.Condition (D): τ₁ is close to τ₂.

In order to make τ_(i) extremely large under the condition (A), (C₁+C₂)may be made extremely larger than (G₁+G₂) from τ_(i)=(C₁+C₂)/(G₁+G₂).Since C₁ and G₁ are parameters of the oxide semiconductor film 403, C₂needs to be increased in order to decrease the charge Q by the metaloxide film 407. However, when C₂ is increased by ∈₂, Q becomes largeaccording to the equation (2) because C₂=∈₂S/d₂, so that there is acontradiction. In other words, the charge Q cannot be adjusted by τ_(i).

In order to make V₂ close to zero under the condition (B), G₂>>G₁ may besatisfied from the equation (3). Since G₁ is a parameter of the oxidesemiconductor film 403, G₂ needs to be increased in order to decreasethe charge Q by the metal oxide film 407. Specifically, d₂ is decreasedor a material in which σ₂ is small is selected because G₂=σ₂S/d₂.However, when d₂ is decreased, C₂ is increased from C₂=∈₂S/d₂, so that Qis increased as in the case of the condition (A); thus, a decrease in d₂cannot be employed. In addition, when σ₂ is large, the electricconductivity of the metal oxide film 407 is higher than that of theoxide semiconductor film 403, which leads to generation of a leakagecurrent and a short circuit with a high probability; thus, a material inwhich σ₂ is large cannot be employed.

In order to make C₂ extremely small under the condition (C), fromC₂=∈₂S/d₂, d₂ is increased or a material in which ∈₂ is small isselected.

In order to make τ₁ close to τ₂ under the condition (D), since τ₁=∈₁/σ₁and τ₂=∈₂/τ₂, a film which satisfies ∈₁/σ₁≈∈₂/σ₂ may be selected. Thisis equivalent to C₁/G₁≈C₂/G₂.

Consequently, in order to prevent the accumulation of the charge Qefficiently, it is preferable that the thickness (d₂) of the metal oxidefilm 407 be increased or a material whose dielectric constant (∈₂) issmall, preferably a material whose dielectric constant is smaller thanthat of the oxide semiconductor film 403 (for example, a material whosedielectric constant ∈ is 8 to 20 inclusive) be selected as a material ofthe metal oxide film 407. Alternatively, a material whose physicalproperty value is close to that of the oxide semiconductor film ispreferably selected as a material of the metal oxide film so as tosatisfy ∈₁/σ₁≈∈₂/σ₂ (∈₁ is the dielectric constant of the oxidesemiconductor and σ₁ is the electric conductivity of the oxidesemiconductor).

As described above, in the transistor 410 including the metal oxide film407 having a function of preventing electrification, charges can beprevented from being accumulated on the back channel side of the oxidesemiconductor film. Further, even when the back channel side of theoxide semiconductor film is positively charged, a positive charge can beremoved immediately with the metal oxide film provided over a topsurface of the oxide semiconductor film. Furthermore, in the transistor410 including the metal oxide film 407, generation of a parasiticchannel on the back channel side of the oxide semiconductor film 403 canbe prevented. By preventing the generation of a parasitic channel on theback channel side of the oxide semiconductor film in the transistor,variation in the threshold voltage can be suppressed. Consequently,variation in the electric conductivity and the like of the oxidesemiconductor film can be suppressed, whereby the reliability of thetransistor can be improved.

As described above, a semiconductor device including an oxidesemiconductor with stable electric characteristics can be provided.Therefore, a semiconductor device with high reliability can be provided.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in other embodiments.

Embodiment 2

In this embodiment, another embodiment of a method for manufacturing asemiconductor device will be described. The same portion as or a portionhaving a function similar to that in the above embodiment can be formedas in the above embodiment, and the same step as or a step similar tothat in the above embodiment can be performed as in the aboveembodiment, and thus, repetitive description is omitted. In addition,detailed description of the same portion is not repeated.

In this embodiment, an example of performing heat treatment on the oxidesemiconductor film before forming the metal oxide film 407 in contactwith the oxide semiconductor film, in the method for manufacturing thetransistor 410 in Embodiment 1, will be described.

This heat treatment may be performed on the oxide semiconductor filmbefore being processed into the island-shaped oxide semiconductor film,as long as the heat treatment is performed after the formation of theoxide semiconductor film and before the formation of the metal oxidefilm 407, and the heat treatment may be performed before the formationof the source electrode 405 a and the drain electrode 405 b or after theformation of the source electrode 405 a and the drain electrode 405 b.

The heat treatment is performed at a temperature of 250° C. to 650° C.inclusive, preferably 450° C. to 600° C. inclusive. For example, thesubstrate is introduced into an electric furnace which is one of heattreatment apparatuses, and the heat treatment is performed on the oxidesemiconductor film at 450° C. for 1 hour in a nitrogen atmosphere. Afterthe heat treatment, the metal oxide film is preferably formed withoutexposing the substrate to the air so that water or hydrogen can beprevented from entering the oxide semiconductor film.

Further, the heat treatment apparatus is not limited to an electricfurnace, and a device for heating an object to be processed by heatconduction or heat radiation from a heating element such as a resistanceheating element may be used. For example, an RTA apparatus such as aGRTA apparatus or an LRTA apparatus can be used. Note that in the casewhere a GRTA apparatus is used as the heat treatment apparatus, thesubstrate may be heated in an inert gas heated to a high temperature of650° C. to 700° C. because the heat treatment time is short.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is 20 ppm or less,preferably 1 ppm or less, more preferably 10 ppb or less), or a rare gas(argon, helium, or the like). Note that it is preferable that water,hydrogen, or the like be not contained in the atmosphere of nitrogen,oxygen, ultra-dry air, or a rare gas. Alternatively, nitrogen, oxygen,or a rare gas which is introduced into the heat treatment apparatus hasa purity of 6N (99.9999%) or more, preferably 7N (99.99999%) or more(that is, the impurity concentration is 1 ppm or less, preferably 0.1ppm or less).

With this heat treatment, impurities such as moisture or hydrogen in theoxide semiconductor film can be reduced.

Further, when the oxide semiconductor film and the metal oxide filmcontaining oxygen are subjected to heat treatment while being in contactwith each other, oxygen which is one of the main components of the oxidesemiconductor and is reduced in the step of removing impurities, can besupplied from the metal oxide film containing oxygen to the oxidesemiconductor film.

Thus, when the oxide semiconductor film is subjected to the heattreatment before forming the metal oxide film and the heat treatmentafter forming the metal oxide film, an i-type (intrinsic) oxidesemiconductor film or a substantially i-type oxide semiconductor filmfrom which impurities such as moisture or hydrogen are furthereliminated, can be obtained.

Therefore, the transistor including the highly purified oxidesemiconductor film has suppressed variation in the electriccharacteristics and is electrically stable.

Furthermore, in the transistor including the metal oxide film,generation of a parasitic channel on the back channel side of the oxidesemiconductor film can be prevented.

As described above, a semiconductor device including an oxidesemiconductor, which has stable electric characteristics, can beprovided. Therefore, a semiconductor device with high reliability can beprovided.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in other embodiments.

Embodiment 3

A semiconductor device with a display function (also referred to as adisplay device) can be manufactured using the transistor whose exampleis described in Embodiment 1 or 2. Moreover, some or all of drivercircuits which include transistors can be formed over a substrate wherea pixel portion is formed, whereby a system-on-panel can be obtained.

In FIG. 2A, a sealant 4005 is provided so as to surround a pixel portion4002 provided over a first substrate 4001, and the pixel portion 4002 issealed by using a second substrate 4006. In FIG. 2A, a signal linedriver circuit 4003 and a scan line driver circuit 4004 which are formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared are mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. Various signals and potentials aresupplied to the signal line driver circuit 4003 and the scan line drivercircuit 4004 which are separately formed and the pixel portion 4002 fromflexible printed circuits (FPCs) 4018 a and 4018 b.

In FIGS. 2B and 2C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Consequently, the pixel portion 4002 and the scan line drivercircuit 4004 are sealed together with a display element, by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. InFIGS. 2B and 2C, the signal line driver circuit 4003 which is formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared is mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. In FIGS. 2B and 2C, various signals andpotential are supplied to the signal line driver circuit 4003 which isseparately formed, the scan line driver circuit 4004, and the pixelportion 4002 from an FPC 4018.

Although FIGS. 2B and 2C each illustrate the example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, the present invention is not limited to this structure.The scan line driver circuit may be separately formed and then mounted,or only part of the signal line driver circuit or part of the scan linedriver circuit may be separately formed and then mounted.

Note that a connection method of a separately formed driver circuit isnot particularly limited, and a chip on glass (COG) method, a wirebonding method, a tape automated bonding (TAB) method, or the like canbe used. FIG. 2A illustrates the example in which the signal line drivercircuit 4003 and the scan line driver circuit 4004 are mounted by a COGmethod. FIG. 2B illustrates the example in which the signal line drivercircuit 4003 is mounted by a COG method. FIG. 2C illustrates the examplein which the signal line driver circuit 4003 is mounted by a TAB method.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC and the like including acontroller are mounted on the panel.

Note that the display device in this specification means an imagedisplay device, a display device, or a light source (including alighting device). Furthermore, the display device also includes thefollowing modules in its category: a module to which a connector such asan FPC, a TAB tape, or a TCP is attached; a module having a TAB tape ora TCP at the tip of which a printed wiring board is provided; and amodule in which an integrated circuit (IC) is directly mounted on adisplay element by a COG method.

Further, the pixel portion and the scan line driver circuit which areprovided over the first substrate 4001 include a plurality oftransistors, to which the transistor whose example is described inEmbodiment 1 or 2 can be applied.

As a display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by a current or avoltage, and specifically includes, in its category, an inorganicelectroluminescent (EL) element, an organic EL element, and the like.Furthermore, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used.

Embodiments of the semiconductor device will be described with referenceto FIG. 3, FIG. 4, and FIG. 5. FIG. 3, FIG. 4, and FIG. 5 correspond tocross-sectional views along line M-N in FIG. 2B.

As illustrated in FIG. 3, FIG. 4, and FIG. 5, the semiconductor deviceincludes a connection terminal electrode 4015 and a terminal electrode4016, and the connection terminal electrode 4015 and the terminalelectrode 4016 are electrically connected to a terminal included in theFPC 4018 through an anisotropic conductive film 4019.

The connection terminal electrode 4015 is formed using the sameconductive film as a first electrode layer 4030, and the terminalelectrode 4016 is formed using the same conductive film as source anddrain electrodes of transistors 4010 and 4011.

Each of the pixel portion 4002 and the scan line driver circuit 4004which are provided over the first substrate 4001 includes a plurality oftransistors. In FIG. 3, FIG. 4, and FIG. 5, the transistor 4010 includedin the pixel portion 4002 and the transistor 4011 included in the scanline driver circuit 4004 are illustrated as an example. In FIG. 3, ametal oxide film 4020 having a function of preventing electrification isprovided over the transistors 4010 and 4011. In FIG. 4 and FIG. 5, aninsulating layer 4021 is further provided. Note that an insulating film4023 is an insulating film serving as a base film.

In this embodiment, the transistor described in Embodiment 1 or 2 can beapplied to the transistors 4010 and 4011.

In the transistors 4010 and 4011, the oxide semiconductor film is anoxide semiconductor film which is highly purified and from whichimpurities such as hydrogen, moisture, a hydroxyl group, or hydride(also referred to as a hydrogen compound) are intentionally removed.Such an oxide semiconductor film is obtained by performing heattreatment after forming the metal oxide film 4020 stacked over the oxidesemiconductor film.

The oxide semiconductor film and the metal oxide film 4020 containingoxygen are in contact with each other when being subjected to the heattreatment; thus, oxygen which is one of the main components of the oxidesemiconductor and is reduced in the step of removing impurities, can besupplied from the metal oxide film 4020 containing oxygen to the oxidesemiconductor film. Thus, the oxide semiconductor film is more highlypurified to become electrically i-type (intrinsic).

Consequently, variation in the electric characteristics of thetransistors 4010 and 4011 each including the highly purified oxidesemiconductor film is suppressed and the transistors 4010 and 4011 areelectrically stable. As described above, semiconductor devices with highreliability can be provided as the semiconductor devices of thisembodiment illustrated in FIG. 3, FIG. 4, and FIG. 5.

In addition, in the transistor including the metal oxide film having afunction of preventing electrification, generation of a parasiticchannel on the back channel side of the oxide semiconductor film can beprevented. By preventing the generation of a parasitic channel on theback channel side of the oxide semiconductor film in the transistor,variation in the threshold voltage can be suppressed.

In addition, in this embodiment, a conductive layer may be provided overthe metal oxide film so as to overlap with a channel formation region ofthe oxide semiconductor film in the transistor 4011 for the drivercircuit. By providing the conductive layer so as to overlap with thechannel formation region of the oxide semiconductor film, the amount ofchange in the threshold voltage of the transistor 4011 before and afterthe BT test can be further reduced. The potential of the conductivelayer may be the same as or different from that of a gate electrode ofthe transistor 4011. The conductive layer can also function as a secondgate electrode. The potential of the conductive layer may be GND, 0V, orin a floating state.

In addition, the conductive layer functions to block an externalelectric field, that is, to prevent an external electric field(particularly, to block static electricity) from effecting the inside (acircuit portion including a thin film transistor). A blocking functionof the conductive layer can prevent the variation in the electriccharacteristics of the transistor due to the effect of an externalelectric field such as static electricity.

The transistor 4010 included in the pixel portion 4002 is electricallyconnected to a display element to form a display panel. A variety ofdisplay elements can be used as the display element as long as displaycan be performed.

Note that an example of a liquid crystal display device using a liquidcrystal element as a display element is illustrated in FIG. 3. In FIG.3, a liquid crystal element 4013 which is a display element includes thefirst electrode layer 4030, a second electrode layer 4031, and a liquidcrystal layer 4008. An insulating film 4032 and an insulating film 4033which serve as alignment films are provided so that the liquid crystallayer 4008 is interposed therebetween. The second electrode layer 4031is provided on the second substrate 4006 side, and the first electrodelayer 4030 and the second electrode layer 4031 are stacked, with theliquid crystal layer 4008 interposed therebetween.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating film and is provided in order tocontrol the thickness of the liquid crystal layer 4008 (a cell gap).Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on conditions.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while the temperature of cholestericliquid crystal is increased. Since the blue phase appears only in anarrow temperature range, a liquid crystal composition in which 5 wt %or more of a chiral material is mixed is used for the liquid crystallayer in order to improve the temperature range. The liquid crystalcomposition which includes liquid crystal exhibiting a blue phase and achiral material has a short response time of 1 msec or less, has opticalisotropy, which makes the alignment process unneeded, and has a smallviewing angle dependence. In addition, since an alignment film does notneed to be provided and thus rubbing treatment is unnecessary,electrostatic discharge damage caused by rubbing treatment can beprevented, and defects and damage of the liquid crystal display devicecan be reduced in the manufacturing process. Thus, the productivity ofthe liquid crystal display device can be increased. A transistorincluding an oxide semiconductor film has a possibility that theelectric characteristics may vary significantly by the influence ofstatic electricity and deviate from the designed range. Therefore, it ismore effective to use a liquid crystal material exhibiting a blue phasefor a liquid crystal display device including a transistor that includesan oxide semiconductor film.

The specific resistivity of the liquid crystal material is 1×10⁹ Ω·cm ormore, preferably 1×10¹¹ Ω·cm or more, more preferably 1×10¹² Ω·cm ormore. The value of the specific resistivity in this specification ismeasured at 20° C.

The size of a storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current of the transistor providedin the pixel portion or the like so that charges can be held for apredetermined period. By using the transistor including the high-purityoxide semiconductor film, it is enough to provide a storage capacitorhaving a capacitance that is ⅓ or less, preferably ⅕ or less of a liquidcrystal capacitance of each pixel.

In the transistor used in this embodiment, which includes the highlypurified oxide semiconductor film, the current in an off state (theoff-state current) can be made small. Accordingly, an electric signalsuch as an image signal can be held for a longer period, and a writinginterval can be set long in an on state. Accordingly, frequency ofrefresh operation can be reduced, which leads to an effect ofsuppressing power consumption.

In addition, the transistor including the highly purified oxidesemiconductor film used in this embodiment can have relatively highfield-effect mobility and thus can operate at high speed. Therefore, byusing the transistor in the pixel portion of the liquid crystal displaydevice, a high-quality image can be provided. In addition, since thedriver circuit portion and the pixel portion which include thetransistor can be formed over one substrate, the number of components ofthe liquid crystal display device can be reduced.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modemay be used. The vertical alignment mode is a method of controllingalignment of liquid crystal molecules of a liquid crystal display panel,in which liquid crystal molecules are aligned vertically to a panelsurface when no voltage is applied. Some examples are given as thevertical alignment mode. For example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, andthe like can be given. Moreover, it is possible to use a method calleddomain multiplication or multi-domain design, in which a pixel isdivided into some regions (subpixels) and molecules are aligned indifferent directions in different regions.

In addition, in the display device, a black matrix (a light-blockinglayer), an optical member (an optical substrate) such as a polarizingmember, a retardation member, or an anti-reflection member, and the likeare provided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

In addition, it is possible to employ a time-division display method(also called a field-sequential driving method) with the use of aplurality of light-emitting diodes (LEDs) as a backlight. By employing afield-sequential driving method, color display can be performed withoutusing a color filter.

As a display method for the pixel portion, a progressive method, aninterlace method, or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite); or R, G, B, and one or more of yellow, cyan, magenta, and thelike can be used. Further, the sizes of display regions may be differentbetween dots of respective color elements. This embodiment is notlimited to the application to a display device for color display but canalso be applied to a display device for monochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. The carriers (electrons and holes) are recombined,and thus, the light-emitting organic compound is excited. Thelight-emitting organic compound returns to a ground state from theexcited state, thereby emitting light. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer in which particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure in which a light-emitting layer is sandwiched betweendielectric layers, which are further sandwiched between electrodes, andits light emission mechanism is localized type light emission thatutilizes inner-shell electron transition of metal ions. Note that thecase where an organic EL element is used as a light-emitting element isdescribed here.

In order to extract light emitted from the light-emitting element, it isacceptable as long as at least one of a pair of electrodes istransparent. A transistor and a light-emitting element are formed over asubstrate. The light-emitting element can have a top emission structurein which light emission is extracted through the surface opposite to thesubstrate; a bottom emission structure in which light emission isextracted through the surface on the substrate side; or a dual emissionstructure in which light emission is extracted through the surfaceopposite to the substrate and the surface on the substrate side. Alight-emitting element having any of these emission structures can beused.

FIG. 4 illustrates an example of a light-emitting device in which alight-emitting element is used as a display element. A light-emittingelement 4513 which is a display element is electrically connected to thetransistor 4010 provided in the pixel portion 4002. A structure of thelight-emitting element 4513 is not limited to the stacked-layerstructure including the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031,which is illustrated in FIG. 4. The structure of the light-emittingelement 4513 can be changed as appropriate depending on a direction inwhich light is extracted from the light-emitting element 4513, or thelike.

A partition wall 4510 is formed using an organic insulating material oran inorganic insulating material. It is particularly preferable that thepartition wall 4510 be formed using a photosensitive resin material tohave an opening over the first electrode layer 4030 so that a sidewallof the opening is formed as a tilted surface with continuous curvature.

The electroluminescent layer 4511 may be formed using a single layer ora plurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 andthe partition wall 4510 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4513. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed. In addition, in aspace which is formed with the first substrate 4001, the secondsubstrate 4006, and the sealant 4005, a filler 4514 is provided forsealing. It is preferable that the light-emitting device be packaged(sealed) with a protective film (such as a laminate film or anultraviolet curable resin film) or a cover material with highair-tightness and little degasification so as not to be exposed to theoutside air, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon, and polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin,a silicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate(EVA) can be used. For example, nitrogen is used for the filler.

In addition, if needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light can bediffused by projections and depressions on the surface so as to reducethe glare can be performed.

Further, an electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also called anelectrophoretic display device (electrophoretic display) and hasadvantages in that it has the same level of readability as regularpaper, it has less power consumption than other display devices, and itcan be set to have a thin and light form.

An electrophoretic display device can have various modes. Anelectrophoretic display device includes a plurality of microcapsulesdispersed in a solvent or a solute, each microcapsule including firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the particles in the microcapsules move in opposite directions to eachother and only the color of the particles gathering on one side isdisplayed. Note that the first particles and the second particles eachcontain pigment and do not move without an electric field. Moreover, thefirst particles and the second particles have different colors (whichmay be colorless).

Thus, an electrophoretic display device is a display that utilizes aso-called dielectrophoretic effect by which a substance having a highdielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have a pigment, color display canalso be achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed of a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed of a composite material thereof.

As the electronic paper, a display device using a twisting ball displaysystem can be used. The twisting ball display system refers to a methodin which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control orientation of the spherical particles, sothat display is performed.

FIG. 5 illustrates an active matrix electronic paper as an embodiment ofthe semiconductor device. The electronic paper in FIG. 5 is an exampleof a display device using a twisting ball display system.

Between the first electrode layer 4030 connected to the transistor 4010and the second electrode layer 4031 provided on the second substrate4006, spherical particles 4613 each of which includes a black region4615 a, a white region 4615 b, and a cavity 4612 which is filled withliquid around the black region 4615 a and the white region 4615 b, areprovided. A space around the spherical particles 4613 is filled with afiller 4614 such as a resin. The second electrode layer 4031 correspondsto a common electrode (counter electrode). The second electrode layer4031 is electrically connected to a common potential line.

In FIG. 3, FIG. 4, and FIG. 5, as the first substrate 4001 and thesecond substrate 4006, flexible substrates, for example, plasticsubstrates having a light-transmitting property or the like can be used,in addition to glass substrates. As plastic, a fiberglass-reinforcedplastic (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film,or an acrylic resin film can be used. In addition, a sheet with astructure in which an aluminum foil is sandwiched between PVF films orpolyester films can be used.

In addition, the metal oxide film 4020 has a function of supplying theoxide semiconductor film with oxygen which is reduced in the step ofremoving impurities such as hydrogen, moisture, a hydroxyl group, orhydride as well as a function of preventing generation of a parasiticchannel on the back channel side of the oxide semiconductor film.

The metal oxide film 4020 may be formed using a gallium oxide filmformed by a sputtering method. Alternatively, the metal oxide film 4020may be a film obtained by adding indium or zinc to gallium oxide, forexample; a gallium oxide film containing indium or zinc at 0.01 at. % to5 at. % can be used. By addition of indium or zinc, the electricconductivity of the metal oxide film 4020 can be improved, wherebyaccumulation of charges can be further reduced.

The insulating layer 4021 can be formed using an inorganic insulatingmaterial or an organic insulating material. Note that the insulatinglayer 4021 formed using a heat-resistant organic insulating materialsuch as an acrylic resin, polyimide, a benzocyclobutene resin,polyamide, or an epoxy resin is preferably used as a planarizinginsulating film. Other than such organic insulating materials, it ispossible to use a low-dielectric constant material (a low-k material), asiloxane based resin, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), or the like. The insulating layer may be formed bystacking a plurality of insulating films formed of these materials.

There is no particular limitation on the method for forming theinsulating layer 4021, and the insulating layer 4021 can be formed,depending on the material, by a sputtering method, a spin coatingmethod, a dipping method, spray coating, a droplet discharge method(e.g., an inkjet method, screen printing, or offset printing), rollcoating, curtain coating, knife coating, or the like.

The display device displays an image by transmitting light from a lightsource or a display element. Therefore, the substrate and the thin filmssuch as the insulating film and the conductive film provided for thepixel portion where light is transmitted have light-transmittingproperties with respect to light in the visible-light wavelength range.

The first electrode layer 4030 and the second electrode layer 4031 (eachof which is also called a pixel electrode layer, a common electrodelayer, a counter electrode layer, or the like) for applying voltage tothe display element may have light-transmitting properties orlight-reflecting properties, which depends on the direction in whichlight is extracted, the position where the electrode layer is provided,the pattern structure of the electrode layer, and the like.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide containing tungsten oxide, indium zinc oxide containing tungstenoxide, indium oxide containing titanium oxide, indium tin oxidecontaining titanium oxide, indium tin oxide (hereinafter referred to asITO), indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed of one or more kinds of materials selected from metals such astungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), andsilver (Ag); alloys of these metals; and nitrides of these metals.

A conductive composition containing a conductive high molecule (alsoreferred to as a conductive polymer) can be used for the first electrodelayer 4030 and the second electrode layer 4031. As the conductive highmolecule, a so-called m-electron conjugated conductive polymer can beused. For example, polyaniline or a derivative thereof, polypyrrole or aderivative thereof, polythiophene or a derivative thereof, a copolymerof two or more of aniline, pyrrole, and thiophene or a derivativethereof, and the like can be given.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protective circuit is preferably formed using anonlinear element.

By using any of the transistors described in Embodiment 1 or 2 asdescribed above, the semiconductor device can have a variety offunctions.

Embodiment 4

A semiconductor device having an image sensor function for reading dataof an object can be formed with the use of the transistor whose exampleis described in Embodiment 1 or 2.

An example of the semiconductor device having an image sensor functionis illustrated in FIG. 6A. FIG. 6A is an equivalent circuit of a photosensor and FIG. 6B is a cross-sectional view illustrating part of thephoto sensor.

One electrode of a photodiode 602 is electrically connected to aphotodiode reset signal line 658, and the other electrode of thephotodiode 602 is electrically connected to a gate of a transistor 640.One of a source and a drain of the transistor 640 is electricallyconnected to a photo sensor reference signal line 672, and the other ofthe source and the drain of the transistor 640 is electrically connectedto one of a source and a drain of a transistor 656. A gate of thetransistor 656 is electrically connected to a gate signal line 659, andthe other of the source and the drain of the transistor 656 iselectrically connected to a photo sensor output signal line 671.

Note that in circuit diagrams in this specification, a transistorincluding an oxide semiconductor film is denoted by a symbol “OS” sothat it can be identified as a transistor including an oxidesemiconductor film. The transistor 640 and the transistor 656 in FIG. 6Aare transistors each including an oxide semiconductor film.

FIG. 6B is a cross-sectional view of the photodiode 602 and thetransistor 640 in the photo sensor. The photodiode 602 functioning as asensor and the transistor 640 are provided over a substrate 601 (a TFTsubstrate) having an insulating surface. A substrate 613 is providedover the photodiode 602 and the transistor 640 using an adhesive layer608.

A metal oxide film 631 having a function of preventing electrification,an interlayer insulating layer 633, and an interlayer insulating layer634 are provided over the transistor 640. The photodiode 602 is providedover the interlayer insulating layer 633. In the photodiode 602, a firstsemiconductor layer 606 a, a second semiconductor layer 606 b, and athird semiconductor layer 606 c are stacked in that order over theinterlayer insulating layer 633 between an electrode layer 641 formedover the interlayer insulating layer 633 and an electrode layer 642formed over the interlayer insulating layer 634.

In the transistor 640, the oxide semiconductor film is an oxidesemiconductor film which is highly purified and from which impuritiessuch as hydrogen, moisture, a hydroxyl group, or hydride (also referredto as a hydrogen compound) are intentionally removed by performing heattreatment.

The oxide semiconductor film and the metal oxide film 631 containingoxygen are in contact with each other when being subjected to the heattreatment; thus, oxygen which is one of the main components of the oxidesemiconductor and is reduced in the step of removing impurities, can besupplied from the metal oxide film 631 containing oxygen to the oxidesemiconductor film. Thus, the oxide semiconductor film is more highlypurified to become electrically i-type (intrinsic).

Consequently, variation in the electric characteristics of thetransistor 640 including the highly purified oxide semiconductor film issuppressed and the transistor 640 is electrically stable. As describedabove, a semiconductor device with high reliability can be provided asthe semiconductor device in this embodiment.

The electrode layer 641 is electrically connected to a conductive layer643 which is formed in the interlayer insulating layer 634, and theelectrode layer 642 is electrically connected to a gate electrode 645through an electrode layer 644. The gate electrode 645 is electricallyconnected to a gate electrode of the transistor 640, and the photodiode602 is electrically connected to the transistor 640.

Here, a pin photodiode in which a semiconductor layer having p-typeconductivity as the first semiconductor layer 606 a, a high-resistancesemiconductor layer (i-type semiconductor layer) as the secondsemiconductor layer 606 b, and a semiconductor layer having n-typeconductivity as the third semiconductor layer 606 c are stacked isillustrated as an example.

The first semiconductor layer 606 a is a p-type semiconductor layer andcan be formed using an amorphous silicon film containing an impurityelement imparting p-type conductivity. The first semiconductor layer 606a is formed by a plasma CVD method with the use of a semiconductorsource gas containing an impurity element belonging to Group 13 (such asboron (B)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with the use of adiffusion method or an ion implantation method. Heating or the like maybe conducted after introducing the impurity element by an ionimplantation method or the like in order to diffuse the impurityelement. In this case, as a method for forming the amorphous siliconfilm, an LPCVD method, a vapor deposition method, a sputtering method,or the like may be used. The first semiconductor layer 606 a ispreferably formed to have a thickness of 10 nm to 50 nm inclusive.

The second semiconductor layer 606 b is an i-type semiconductor layer(intrinsic semiconductor layer) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor layer 606 b, anamorphous silicon film is formed with the use of a semiconductor sourcegas by a plasma CVD method. As the semiconductor source gas, silane(SiH₄) may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄,or the like may be used. The second semiconductor layer 606 b may beformed by an LPCVD method, a vapor deposition method, a sputteringmethod, or the like. The second semiconductor layer 606 b is preferablyformed to have a thickness of 200 nm to 1000 nm inclusive.

The third semiconductor layer 606 c is an n-type semiconductor layer andis formed using an amorphous silicon film containing an impurity elementimparting n-type conductivity. The third semiconductor layer 606 c isformed by a plasma CVD method with the use of a semiconductor source gascontaining an impurity element belonging to Group 15 (e.g., phosphorus(P)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with the use of adiffusion method or an ion implantation method. Heating or the like maybe conducted after introducing the impurity element by an ionimplantation method or the like in order to diffuse the impurityelement. In this case, as a method for forming the amorphous siliconfilm, an LPCVD method, a vapor deposition method, a sputtering method,or the like may be used. The third semiconductor layer 606 c ispreferably formed to have a thickness of 20 nm to 200 nm inclusive.

The first semiconductor layer 606 a, the second semiconductor layer 606b, and the third semiconductor layer 606 c are not necessarily formedusing an amorphous semiconductor, and they may be formed using apolycrystalline semiconductor or a microcrystalline semiconductor (asemiamorphous semiconductor: SAS).

Considering Gibbs free energy, a microcrystalline semiconductor is in ametastable state which is intermediate between an amorphous state and asingle crystal state. That is, the microcrystalline semiconductor is asemiconductor having a third state which is stable in terms of freeenergy and has a short range order and lattice distortion. Columnar-likeor needle-like crystals grow in a normal direction with respect to asubstrate surface. The Raman spectrum of microcrystalline silicon, whichis a typical example of a microcrystalline semiconductor, is located inlower wave numbers than 520 cm⁻¹, which represents a peak of the Ramanspectrum of single crystal silicon. That is, the peak of the Ramanspectrum of the microcrystalline silicon exists between 520 cm⁻¹ whichrepresents single crystal silicon and 480 cm⁻¹ which representsamorphous silicon. The semiconductor contains hydrogen or halogen of atleast 1 at. % to terminate a dangling bond. Moreover, microcrystallinesilicon contains a rare gas element such as helium, argon, krypton, orneon to further promote lattice distortion, so that stability can beincreased and a favorable microcrystalline semiconductor film can beobtained.

The microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens ofmegahertz to several hundreds of megahertz or using a microwave plasmaCVD apparatus with a frequency of 1 GHz or more. Typically, themicrocrystalline semiconductor film can be formed using a gas containingsilicon such as SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, or SiF₄, which isdiluted with hydrogen. Further, with the gas containing silicon dilutedwith one or plural kinds of rare gas elements selected from helium,argon, krypton, and neon in addition to hydrogen, the microcrystallinesemiconductor film can be formed. In that case, the flow ratio ofhydrogen to the gas containing silicon is 5:1 to 200:1 inclusive,preferably 50:1 to 150:1 inclusive, more preferably 100:1. Further, ahydrocarbon gas such as CH₄ or C₂H₆, a gas containing germanium such asGeH₄ or GeF₄, F₂, or the like may be mixed into the gas containingsilicon.

In addition, since the mobility of holes generated by the photoelectriceffect is lower than that of electrons, the pin photodiode has bettercharacteristics when a surface on the p-type semiconductor layer side isused as a light-receiving surface. Here, an example in which light 622received by the photodiode 602 from a surface of the substrate 601, overwhich the pin photodiode is formed, is converted into electric signalsis described. Further, light from the semiconductor layer having aconductivity type opposite to that of the semiconductor layer on thelight-receiving surface is disturbance light; therefore, the electrodelayer on that side is preferably formed using a light-blockingconductive film. Note that a surface of the n-type semiconductor layerside can alternatively be used as the light-receiving surface.

The metal oxide film 631 may be formed using a gallium oxide film formedby a sputtering method. In addition, the metal oxide film 631 may be afilm obtained by adding indium or zinc to gallium oxide; for example, agallium oxide film containing indium or zinc at 0.01 at. % to 5 at. %can be used. By addition of indium or zinc, the electric conductivity ofthe metal oxide film 631 can be improved, whereby accumulation ofcharges can be further reduced.

For reduction of the surface roughness, an insulating layer functioningas a planarizing insulating film is preferably used as the interlayerinsulating layers 633 and 634. The interlayer insulating layers 633 and634 can be formed using, for example, an organic insulating materialsuch as polyimide, an acrylic resin, a benzocyclobutene resin,polyamide, or an epoxy resin. Other than such organic insulatingmaterials, a single-layer or stacked-layer structure using alow-dielectric constant material (a low-k material), a siloxane basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like can be used.

The interlayer insulating layer 633 and the interlayer insulating layer634 can be formed using an insulating material by a sputtering method, aspin coating method, a dipping method, spray coating, a dropletdischarge method (e.g., an inkjet method, screen printing, or offsetprinting), roll coating, curtain coating, knife coating, or the likedepending on the material.

When the light 622 that enters the photodiode 602 is detected, data onan object to be detected can be read. Note that a light source such as abacklight can be used at the time of reading data on an object to bedetected.

The transistor whose example is described in Embodiment 1 or 2 can beused as the transistor 640. The transistor including the oxidesemiconductor film which is highly purified by intentionally removingimpurities such as hydrogen, moisture, a hydroxyl group, or hydride(also referred to as a hydrogen compound) has a suppressed variation inthe electric characteristics and is electrically stable. In addition, inthe transistor including the metal oxide film having a function ofpreventing electrification, generation of a parasitic channel on theback channel side of the oxide semiconductor film can be prevented. Bypreventing the generation of a parasitic channel on the back channelside of the oxide semiconductor film in the transistor, variation in thethreshold voltage can be suppressed. Therefore, a semiconductor devicewith high reliability can be provided.

This embodiment can be implemented in appropriate combination with thestructures described in other embodiments.

Embodiment 5

The liquid crystal display device disclosed in this specification can beapplied to a variety of electronic devices (including game machines).Examples of the electronic devices are a television set (also referredto as a television or a television receiver), a monitor of a computer orthe like, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, apersonal digital assistant, an audio reproducing device, a large-sizedgame machine such as a pachinko machine, and the like. Examples of theelectronic devices each including the liquid crystal display devicedescribed in the above embodiment will be described.

FIG. 7A illustrates an electronic book reader (also referred to as ane-book reader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar cell 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader illustrated in FIG. 7Ahas a function of displaying various kinds of data (e.g., a still image,a moving image, and a text image) on the display portion, a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a function of operating or editing the data displayed on thedisplay portion, a function of controlling processing by various kindsof software (programs), and the like. Note that in FIG. 7A, the chargeand discharge control circuit 9634 has a battery 9635 and a DCDCconverter (hereinafter, abbreviated as a converter) 9636 as an example.The semiconductor device described in any of Embodiments 1 to 4 can beapplied to the display portion 9631, whereby a highly reliableelectronic book reader can be provided.

In the case of using a transflective or reflective liquid crystaldisplay device as the display portion 9631 in the structure illustratedin FIG. 7A, the electronic book reader may be used in a comparativelybright environment. In that case, power generation by the solar cell9633 and charge by the battery 9635 can be effectively performed, whichis preferable. Since the solar cell 9633 can be provided on a space (asurface or a rear surface) of the housing 9630 as appropriate, thebattery 9635 can be efficiently charged, which is preferable. When alithium ion battery is used as the battery 9635, there is an advantageof downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 7A are described with reference to ablock diagram in FIG. 7B. The solar cell 9633, the battery 9635, theconverter 9636, a converter 9637, switches SW1 to SW3, and the displayportion 9631 are illustrated in FIG. 7B, and the battery 9635, theconverter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634.

First, an example of the operation in the case where power is generatedby the solar cell 9633 using external light is described. The voltage ofpower generated by the solar cell 9633 is raised or lowered by theconverter 9636 so as to be voltage for charging the battery 9635. Then,when the power from the solar cell 9633 is used for the operation of thedisplay portion 9631, the switch SW1 is turned on and the voltage of thepower is raised or lowered by the converter 9637 so as to be a voltageneeded for the display portion 9631. In addition, when display on thedisplay portion 9631 is not performed, the switch SW1 is turned off andthe switch SW2 is turned on so that charge of the battery 9635 may beperformed.

Next, the operation in the case where power is not generated by thesolar cell 9633 using external light is described. The voltage of powerstored in the battery 9635 is raised or lowered by the converter 9637 byturning on the switch SW3. Then, power from the battery 9635 is used forthe operation of the display portion 9631.

Note that although the solar cell 9633 is described as an example of ameans for charge, charge of the battery 9635 may be performed withanother means. In addition, a combination of the solar cell 9633 andanother means for charge may be used.

FIG. 8A illustrates a laptop personal computer, which includes a mainbody 3001, a housing 3002, a display portion 3003, a keyboard 3004, andthe like. The semiconductor device described in any of Embodiments 1 to4 is applied to the display portion 3003, whereby a highly reliablelaptop personal computer can be provided.

FIG. 8B is a personal digital assistant (PDA) including a displayportion 3023, an external interface 3025, an operation button 3024, andthe like in a main body 3021. In addition, a stylus 3022 is included asan accessory for operation. The semiconductor device described in any ofEmbodiments 1 to 4 is applied to the display portion 3023, whereby ahighly reliable personal digital assistant (PDA) can be provided.

FIG. 8C illustrates an example of an electronic book reader. Forexample, an electronic book reader includes two housings, a housing 2701and a housing 2703. The housing 2701 and the housing 2703 are combinedwith a hinge 2711 so that the electronic book reader can be opened andclosed with the hinge 2711 as an axis. With such a structure, theelectronic book reader can operate like a paper book.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the structure where different images are displayed in theabove display portions, for example, the right display portion (thedisplay portion 2705 in FIG. 8C) can display text and the left displayportion (the display portion 2707 in FIG. 8C) can display images. Thesemiconductor device described in any of Embodiments 1 to 4 is appliedto the display portion 2705 and the display portion 2707, whereby ahighly reliable electronic book reader can be provided.

FIG. 8C illustrates the example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, operation keys 2723, a speaker 2725,and the like. With the operation keys 2723, pages can be turned. Notethat a keyboard, a pointing device, or the like may be provided on thesurface of the housing, on which the display portion is provided.Furthermore, an external connection terminal (an earphone terminal, aUSB terminal, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the electronic book reader may have a function of anelectronic dictionary.

The electronic book reader may have a configuration capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, desired book data or the like can be purchased anddownloaded from an electronic book server.

FIG. 8D illustrates a mobile phone, which includes two housings, ahousing 2800 and a housing 2801. The housing 2801 includes a displaypanel 2802, a speaker 2803, a microphone 2804, a pointing device 2806, acamera lens 2807, an external connection terminal 2808, and the like. Inaddition, the housing 2800 includes a solar cell 2810 for charging themobile phone, an external memory slot 2811, and the like. Further, anantenna is incorporated in the housing 2801. The semiconductor devicedescribed in any of Embodiments 1 to 4 is applied to the display panel2802, whereby a highly reliable mobile phone can be provided.

The display panel 2802 is provided with a touch panel. A plurality ofoperation keys 2805 which are displayed as images are illustrated bydashed lines in FIG. 8D. Note that a boosting circuit by which a voltageoutput from the solar cell 2810 is increased to be sufficiently high foreach circuit is also included.

In the display panel 2802, the display direction can be appropriatelychanged depending on a usage pattern. Further, the mobile phone isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus it can be used as a video phone. The speaker 2803and the microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Moreover, thehousing 2800 and the housing 2801 developed as illustrated in FIG. 8Dcan be slid so that one is lapped over the other; thus, the size of themobile phone can be reduced, which makes the mobile phone suitable forbeing carried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer or the like are possible.Moreover, a large amount of data can be stored by inserting a storagemedium into the external memory slot 2811 and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 8E illustrates a digital video camera which includes a main body3051, a display portion A 3057, an eyepiece 3053, an operation switch3054, a display portion B 3055, a battery 3056, and the like. Thesemiconductor device described in any of Embodiments 1 to 4 is appliedto the display portion A 3057 and the display portion B 3055, whereby ahighly reliable digital video camera can be provided.

FIG. 8F illustrates an example of a television set. In a television set9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605. The semiconductor device described in any ofEmbodiments 1 to 4 is applied to the display portion 9603, whereby ahighly reliable television set can be provided.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) data communication can beperformed.

This embodiment can be implemented in appropriate combination with thestructures described in other embodiments.

This application is based on Japanese Patent Application serial no.2010-072256 filed with Japan Patent Office on Mar. 26, 2010, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a gate electrode; anoxide semiconductor film including a channel formation region; a gateinsulating film between the gate electrode and the oxide semiconductorfilm; a source electrode and a drain electrode over the oxidesemiconductor film; and a metal oxide film over the oxide semiconductorfilm, the source electrode and the drain electrode, wherein the metaloxide film is in contact with part of the oxide semiconductor film,wherein the oxide semiconductor film contains indium, gallium and zinc,and wherein the metal oxide film contains gallium and indium.
 3. Thesemiconductor device according to claim 2, wherein a thickness of themetal oxide film is larger than a thickness of the oxide semiconductorfilm.
 4. The semiconductor device according to claim 2, wherein aconcentration of indium in the metal oxide film is 0.01 at. % to 5 at.%.
 5. The semiconductor device according to claim 2, wherein adifference between a band gap of the metal oxide film and a band gap ofthe oxide semiconductor film is less than 3.0 eV.
 6. A display devicecomprising the semiconductor device according to claim
 2. 7. Asemiconductor device comprising: a gate electrode; an oxidesemiconductor film including a channel formation region; a gateinsulating film between the gate electrode and the oxide semiconductorfilm; a source electrode and a drain electrode over the oxidesemiconductor film; and a metal oxide film over the oxide semiconductorfilm, the source electrode and the drain electrode, wherein the metaloxide film is in contact with part of the oxide semiconductor film,wherein the oxide semiconductor film contains indium, gallium and zinc,and wherein the metal oxide film contains gallium and zinc.
 8. Thesemiconductor device according to claim 7, wherein a thickness of themetal oxide film is larger than a thickness of the oxide semiconductorfilm.
 9. The semiconductor device according to claim 7, wherein aconcentration of zinc in the metal oxide film is 0.01 at. % to 5 at. %.10. The semiconductor device according to claim 7, wherein a differencebetween a band gap of the metal oxide film and a band gap of the oxidesemiconductor film is less than 3.0 eV.
 11. A display device comprisingthe semiconductor device according to claim
 7. 12. A semiconductordevice comprising: a gate electrode; an oxide semiconductor filmincluding a channel formation region; a gate insulating film between thegate electrode and the oxide semiconductor film; a source electrode anda drain electrode over the oxide semiconductor film; and a metal oxidefilm over the oxide semiconductor film, the source electrode and thedrain electrode, wherein the metal oxide film is in contact with part ofthe oxide semiconductor film, wherein the oxide semiconductor filmcontains indium and gallium, wherein the metal oxide film containsgallium and indium, and wherein a concentration of indium in the metaloxide film is different from a concentration of indium in the oxidesemiconductor film.
 13. The semiconductor device according to claim 12,wherein a thickness of the metal oxide film is larger than a thicknessof the oxide semiconductor film.
 14. The semiconductor device accordingto claim 12, wherein the concentration of indium in the metal oxide filmis 0.01 at. % to 5 at. %.
 15. The semiconductor device according toclaim 12, wherein a difference between a band gap of the metal oxidefilm and a band gap of the oxide semiconductor film is less than 3.0 eV.16. A display device comprising the semiconductor device according toclaim
 12. 17. A semiconductor device comprising: a gate electrode; anoxide semiconductor film including a channel formation region; a gateinsulating film between the gate electrode and the oxide semiconductorfilm; a source electrode and a drain electrode over the oxidesemiconductor film; and a metal oxide film over the oxide semiconductorfilm, the source electrode and the drain electrode, wherein the metaloxide film is in contact with part of the oxide semiconductor film,wherein the oxide semiconductor film contains gallium and zinc, whereinthe metal oxide film contains gallium and zinc, and wherein aconcentration of zinc in the metal oxide film is different from aconcentration of zinc in the oxide semiconductor film.
 18. Thesemiconductor device according to claim 17, wherein a thickness of themetal oxide film is larger than a thickness of the oxide semiconductorfilm.
 19. The semiconductor device according to claim 17, wherein theconcentration of zinc in the metal oxide film is 0.01 at. % to 5 at. %.20. The semiconductor device according to claim 17, wherein a differencebetween a band gap of the metal oxide film and a band gap of the oxidesemiconductor film is less than 3.0 eV.
 21. A display device comprisingthe semiconductor device according to claim 17.